1. Field of the Invention
This invention relates to a toner used in recording processes utilizing electrophotography, electrostatic recording, magnetic recording, toner-jet recording or the like. More particularly, this invention relates to a toner for developing an electrostatically charged image used in copying machines, printers and facsimile machines in which a toner image is previously formed on an electrostatic latent image bearing member and thereafter the toner image is transferred to a transfer medium to form an image, and also relates to a two-component developer and an image forming method which make use of the toner.
2. Related Background Art
Methods are conventionally well known in which a dry-process developer as an agent for rendering latent images visible is carried on the surface of a developer carrying member, the developer is transported and supplied to the vicinity of the surface of a latent image bearing member holding an electrostatic latent image thereon and the electrostatic latent image is developed by a toner of the developer while applying an alternating electric field across the latent image bearing member and the developer carrying member, to render the electrostatic latent image visible.
The developer carrying member is often called "developing sleeve" in the following description because developing sleeves are commonly in wide use as the developer carrying member. The latent image bearing member (photosensitive member) is also often called "photosensitive drum" in the following description because photosensitive drums are commonly in wide use as the latent image bearing member.
As the above developing method, so called magnetic-brush development method is conventionally known in which a magnetic brush is formed on the surface of a developing sleeve internally provided with a magnet, by the use of, e.g., a developer (two-component developer) comprised of two components (carrier particles and toner particles), the magnetic brush thus formed is rubbed with, or brought close to, a photosensitive drum set opposingly to the developing sleeve while keeping a minute development gap between them, and an alternating electric field is continuously applied across the developing sleeve and the photosensitive drum (between S-D) to repeatedly cause the toner particles to transit from the developing sleeve side to the photosensitive drum side and vice versa, to carry out development (see, e.g., Japanese Patent Application Laid-Open No. 55-32060 and No. 59-165082).
In such a magnetic brush development method making use of a two-component developer, the toner particles are triboelectrically charged by mixing them with carrier particles. Since the carrier particles have a higher specific gravity than the toner particles, the toner particles undergo a high mechanical strain because of their friction with the carrier particles when mixed, so that the deterioration of toner tends to accelerate with the progress of development operated repeatedly.
Once such deterioration of toner has occurred, it may cause concretely the phenomena that the density of fixed images changes as a result of long-term service, that the toner particles adhere to non-image areas to cause what is so-called "fog" and that the minute-image reproducibility becomes poor.
In the electrophotographic process, after the toner image formed on the photosensitive drum has been transferred to the transfer medium, the toner remaining on the photosensitive drum without being transferred to the transfer medium is removed from the surface of the photosensitive drum by a cleaning means in the step of cleaning and is collected. Blade cleaning, fur brush cleaning or roller cleaning are used as the cleaning means.
When, however, the toner on the photosensitive drum is removed and collected by using the cleaning means, from the aspect of apparatus the apparatus must be made larger due to providing such a cleaning means. This has been a bottleneck in attempts to make apparatus compact. Accordingly, image forming apparatus having no cleaning means are desired.
From the viewpoint of ecology, a cleanerless system or toner reuse system that may produce no waste toner is long-awaited in the sense of effective utilization of toners.
Such a technique is known as a technique called cleaning-at-development in which the toner remaining on the photosensitive drum after transfer (transfer residual toner) is collected at the time of development in a developing assembly and the toner collected is again used in the development.
As this technique called "cleaning-at-development" (or "cleanerless") system, for example, Japanese Patent Publication No. 5-69427 discloses that one image is formed at one rotation of the photosensitive drum so that any effect of the transfer residual toner does not appear on the same image. Japanese Patent Application Laid-Open No. 64-20587, No. 2-259784, No. 4-50886 and No. 5-165378 disclose a system in which the transfer residual toner is dispersed or driven off by a drive-off member to make it into non-patterns so that it may hardly appear on images even when the surface of the same photosensitive drum is utilized several times for one image.
Japanese Patent Application Laid-Open No. 5-2287 discloses a system in which a relation of toner charge quantity around the photosensitive drum is specified so that any positive memory or negative memory caused by the transfer residual toner may not appear on images. It, however, does not disclose any specific constitution for how to control the toner charge quantity.
In Japanese Patent Application Laid-Open No. 59-133573, No. 62-203182, No. 63-133179, No. 2-302772, No. 4-155361, No. 5-2289, No. 5-53482 and No. 5-61383, which disclose techniques relating to the cleanerless system, it is proposed, in relation to imagewise exposure, to make exposure using light having a high intensity or to use a toner capable of transmitting light having an exposure wavelength. However, only making exposure intensity higher may cause a blur in dot formation of a latent image itself to cause an insufficient isolated-dot reproducibility, resulting in images having a poor resolution in respect of image quality, in particular, images lacking in gradation in graphic images.
As for the means making use of the toner capable of transmitting light having an exposure wavelength, the transmission of light certainly has a great influence on the fixed toner having been made smooth and having no particle boundary. However, as a mechanism of screening exposure light, it has less effect because it more chiefly concerns the scattering of light on the toner particle surfaces than the coloring of toner itself. Moreover, colorants of toners must be selected in a narrower range, and also at least three types of exposure means having different wavelengths are required when full-color formation is intended. This goes against making apparatus simple, which is one of features of the cleaning-at-development.
In an image forming method employing a contact charging system in which the photosensitive drum which is the member to be charged is primarily charged by injecting charges into it by means of a contact charging member, any faulty charging due to contamination (toner-spent) of the charging member tends to cause faulty images and to cause a problem on running performance. Thus, it has been a pressing need for enabling many-sheet printing to restrain the influence of the faulty charging due to contamination of the charging member.
Examples in which the contact charging system is used in the image forming system employing the cleanerless or cleaning-at-development system are seen in Japanese Patent Application Laid-Open No. 4-234063 and No. 6-230652, which disclose an image forming method in which the cleaning to remove transfer residual toner from the photosensitive drum is also carried out simultaneously in a back-exposure simultaneous developing system.
However, the proposals in these publications are applicable to an image forming method in which charge potential and developing applied bias are formed at low electric fields. In image formation under a higher electric field charging-developing applied bias, which is conventionally widely applied in electrophotographic apparatus, leak may occur to cause faulty images such as lines and spots.
A method is also proposed in which the toner having adhered to the charging member is moved to the photosensitive drum at the time of formation of no image so that any ill effect caused by adhesion of the transfer residual toner can be prevented. However, the proposal does not mention anything about improvement in recovery rate in the developing step, of the toner moved to the photosensitive drum, and about any influence on development that may be caused by the collection of toner in the developing step.
In addition, if the cleaning effect against the transfer residual toner is insufficient at the time of development, there may be caused problems that a positive ghost may appear, since the subsequent toner participates in development on the photosensitive drum on which the transfer residual toner is present and hence an image formed thereat may have a higher density than its surroundings and that, if the transfer residual toner is in a too large quantity, a positive memory may be caused on images, since the toner may not be completely collected at the development part. No fundamental solution of these problems has been achieved.
Light screening caused by the transfer residual toner especially comes into question when the photosensitive drum is repeatedly used on one sheet of transfer medium, i.e., when the length corresponding to one round of the photosensitive drum is smaller than the length in the moving direction of the transfer medium. Since the charging, exposure and development must be made in the state the transfer residual toner is present on the photosensitive drum, the electric potential at the photosensitive drum surface portion where the transfer residual toner is present can not be completely dropped to make development contrast insufficient, which, in reverse development, appears on images as a negative ghost, having a lower density than the surroundings. The photosensitive drum having passed through an electrostatic transfer step stands charged in a polarity reverse to the polarity of toner charge on the whole, where, because of any deterioration of charge injection performance in the photosensitive drum as a result of repeated use, the transfer residual toner not controlled to have the normal charge polarity in the charging member may leak from the charging member during image formation to intercept exposure light, so that latent images are disordered and any desired electric potential cannot be attained, thereby causing a negative memory on images. Such problems may further occur, and it is sought to make fundamental solution of these problems.
In recent years, output instruments such as copying machines and laser beam printers employing the above electrophotographic process have become low-cost and have made a progress in digital techniques. Accordingly, it is required to form high-quality images more faithful to originals by using much image information. Especially when images such as printed photographs, catalogs and maps are copied, it is demanded to reproduce them very finely and faithfully throughout details, without causing crushed line images and broken line images.
In such trends of techniques, toners are sought to have such performance that, in the course of development, transfer and fixing, the toner may cause less scatter of toner around latent images, the toner itself maintains a high charging performance and simultaneously the toner after development can be transferred to the transfer medium at a transfer efficiency of almost 100%.
As means for improving an image quality in the electrophotographic process, the following methods are available: (i) a method in which the latent image on the latent image bearing member is rubbed with ears of developer while keeping dense the rise of ears of developer on the developer carrying member; (ii) a method in which a bias electric field is applied across the developer carrying member and the latent image bearing member to thereby make the toner readily flown; (iii) a method in which the developing assembly itself is made to have a higher agitation performance inside the assembly so that a high chargeability can be permanently maintained; and also (iv) a method in which dot size itself of the latent image is made finer to improve resolution.
Such means concerned with the development are very effective and hold a part of important techniques for achieving a high image quality. However, taking account of more improvement in image quality, the performance of the developer itself is considered to have a great influence.
Especially in the image formation for full-color images, monochromatic toners are used in development and transferred many times, so that toners are formed in multi-layer at the latent image areas, where the layers tend to have a lower electric potential as they come near to the outermost layer, resulting in a difference in developing performance of toners between the lowermost layer and the uppermost layer in some cases.
Further, there cannot only be attained a faithful color reproducibility due to poor color mixing after a heat-melting treatment, but also there may often be caused drawbacks such as lowering of transfer performance and scatter of toner on non-latent-image electric-potential areas.
From the viewpoint of process factors, a great influence of toner performance on the improvement in image quality is considered as stated above. For the purpose of improving image quality, various developers are hitherto proposed. For example, Japanese Patent Application Laid-open No. 51-3244 discloses a non-magnetic toner in which its particle size distribution is controlled so that the image quality can be improved. This toner is composed chiefly of toner particles having a particle diameter of from 8 to 12 .mu.m, which are relatively coarse. According to studies made by the present inventors, it is difficult for the toner with such particle diameter to fly onto latent images in a dense state. Also, the toner, as having the feature that particles with particle diameters of 5 .mu.m or smaller are contained in an amount of not more than 30% by number and particles with particle diameters of 20 .mu.m or larger are contained in an amount of not more than 5% by number, tends to result in a low uniformity because of a broadness of its particle size distribution. In order to form sharp images by the use of the toner comprising such relatively coarse toner particles and having a broad particle size distribution, the toner particles in each layer under the multi-layer configuration as described above must be thickly overlaid so that any spaces between toner particles can be filled up to increase apparent image density. This brings about the problem of an increase in the consumption of toner necessary to attain a given image density.
Japanese Patent Application Laid-Open No. 58-129437 discloses a non-magnetic toner having an average particle diameter of from 6 to 10 .mu.m and being held by particles with particle diameters of 5 to 8 .mu.m in the greatest number. This toner, however, contains particles with particle diameters of 5 .mu.m or smaller in an amount of as small as 15% by number, and tends to form images lacking in sharpness.
As a result of studies made by the present inventors, they have ascertained that toner particles with particle diameters of 5 .mu.m or smaller contribute the clear reproduction of minute dots of latent images and have a chief function to densely lay the toner onto the whole latent image. In particular, electrostatic latent images on a photosensitive drum have a higher electric field intensity at their edges than at their inner sides because of concentrated lines of electric force, and the quality of toner particles gathered at that portions influences the sharpness of an image quality. The studies made by the present inventors have revealed that the control of the quantity of toner particles with particle diameters of 5 .mu.m or smaller is effective for improving a high-light gradation.
However, the toner particles with particle diameters of 5 .mu.m or smaller have a strong adhesion to the surface of the latent image bearing member, so that the transfer residual toner can be removed by cleaning with difficulty. In addition, as a result of continuous printing, some low-electrical-resistance matters such as paper dust or ozonides and the toner may consequently stick to the photosensitive drum.
For the purpose of scraping off such low-electrical-resistance matters and the toner having stuck, Japanese Patent Application Laid-Open No. 60-32060 and No. 60-136752 disclose a proposal to add as an abrasive an inorganic fine powder having a BET specific surface area of from 0.5 to 30 m.sup.2 /g as measured by nitrogen adsorption. This is effective for preventing the toner from sticking, but it is difficult to attain the desired abrasive effect unless the developer is improved in charging stability. Consequently, this has been insufficient for achieving stable cleaning.
Japanese Patent Application Laid-Open No. 61-188546, No. 63-289559 and No. 7-261446 also disclose a proposal of a toner in which two or three kinds of inorganic fine particles are added and mixed in a toner. This, however, chiefly aims at abrasive effect for the purpose of imparting fluidity and removing the matters stuck to the photosensitive drum, and has not attained the effect of greatly improving the transfer performance of the toner. Use of the same kind of inorganic fine particles (of, e.g., silica) may make unstable not only the fluidity-providing effect but also the charge-providing properties of the toner, to cause a possibility of toner scatter and fog. Moreover, the proposal is concerned with only average particle diameter of the inorganic fine particles and is unclear about their particle size distribution. Accordingly, there is also a possibility of causing the sticking of toner to the photosensitive drum.
For the purpose of achieving much higher image quality, Japanese Patent Application Laid-Open No. 2-222966 discloses using fine silica particles and fine alumina particles in combination. However, the fine silica particles have so large a BET specific surface area as to make it difficult to attain any remarkable effect as a spacer between toner particles.